The present invention relates to an artificial foot for a leg prosthesis consisting of a foamed plastic molded foot part with a plate-shaped reinforcing member embedded within its plantar area that is provided with at least two approximately equally long superimposed leaf springs with an elastic distance piece and sliding insert of polyethylene possessing a high slidability and which, within the area of the ball of the foot, is constructed with an offset extending in the same manner as the ball of the foot for supporting the foot rolling function and with--serving in each case as elastically yielding springing--a forefoot core and a function core consisting of a tongue disposed within the calcaneal area, the function core and the reinforcing member being rigidly interconnected.
Polyurethane foamed plastic has been used as material for artificial feet for a long time; it possesses the advantage of a low weight.
In order to enable the artificial foot to approximately perform the function predetermined in a natural foot for instance, in the U.S. Pat. No. 3,335,428, a leg prosthesis foot part has been proposed which is molded from resilient plastic materials of differing hardness.
Furthermore, from the DE-PS No. 354,246, an artificial foot is known in which, within the plantar area of the artificial foot, a metal strip is embedded. This artificial foot is intended to render possible the taking of long strides for a foot or leg amputee since the taking of big strides gives rise to the oblique change of position of the lower leg toward the rear, thus the yielding of the heel of the foot resting on the ground with its entire undersurface. It is further intended to bring about the easy straightening up again of the lower leg without that the same exceeds the vertical position, while the forward shifting of the body weight and the raising of the heel is intended to be rendered possible by the normal flexibility of the metatarsal and dactylar portion of the artificial foot. This flexibility is intended to be achieved by the embedding of a spring plate passing through the plantar and calcaneal portion. In order to now be able to bring about the oblique change of position toward the rear of the lower leg, this known embodiment of an artificial foot provides a connection between the rigid lower leg and the spring plate, in which the lower leg moves rearward from its upright position in the manner of a cradle and, when shifting the body weight forward, returns once more into the first position. This seesaw motion takes place due to the circumstance that the cuneiformly tapering, rigid lower leg is seated in a so-called saddle, one slope of which is formed by the instep of the artificial foot, the counterslope of which is formed by a branch of the leaf spring. For a further shock absorption, a compressible wedge cushion, e.g. of soft rubber, is located between the branch and the actual spring plate. In order to avoid the occurrence of undesired noises when the front wedge surface on the lower leg strikes against the foot, the forefoot or metatarsus consists of a flexibly or readily compressible, but at any rate sound-absorbent, mass, preferably felt. The abutment surface of the lower leg, too, is covered with a cap of this material. In this artificial foot, a steel spring having spring properties is used which extends as far s into the forefoot. However, the important elasticity differences between metatarsus and forefoot have not been taken into constructional consideration in this case. The steel spring employed does not render a natural rolling possible; in addition, the connecting problems between the different materials within the elastic areas are not solved from a structural view.
Moreover, from the DE-PS No. 361 972, there is known an artificial foot with a longitudinal spring system consisting of several leaf springs that are stepped relative to each other, said leaf springs being fastened with their rearward ends to the lower side of a rigid block forming the rear foot portion, but which is separated from the sole by an intermediate layer, while the front ends extend downwardly in an appropriate double bend and press directly on the sole which is expediently protected by a protective plate. In this artificial foot, too, an attempt is made to control the movements of the foot by means of a metal spring, however, it does not allow any mobility or flexibility within the metatarsophalangeal area.
In an artificial foot for leg prostheses known from the U.S. Pat. No. 2,556,525, a rigid but flexible plastic portion is embedded in an external foamed plastic molded part that extends across the entire length of the foot, while a metal insert of spring steel is embedded in said rigid but flexible plastic part. Even if an internal, partially flexible plastic part with a metal insert of spring steel were to be employed in this known artificial foot, it is not possible in this artificial foot to lay the rolling function onto the trisectional line predetermined by nature. Incidentally, the metal insert is also run up to the tip of the foot.
Despite the use of a flexible plastic part and of a metal insert of spring steel, an adequate mobility within the metatarsophalangeal articulation, as exists in nature, is not possible. The important flexing in the metatarsophalangeal joint has not been taken into consideration in this known embodiment and, on account of the constructional configuration, is not possible either.
That is why, in order to provide an artificial foot for leg prostheses with a rolling resistance of high resilience predetermined over a relatively long period and a flexing possibility in the metatarsophalangeal articulation, the artificial foot described in the beginning has been proposed in the DE-PS No. 23 41 887. In this artificial foot, the forefoot elasticity is ensured by the use of a homogeneous Vulkollan (elastomer) constituent, that is to say that, in the forefoot, a restoring or returning resilience is taken into account which went far beyond the padding effects formed until then. However, it had not been possible to functionally utilize the advantages of said restoring resilience in its physical influence value on the gait image.
In order to ensure a great metatarsal elasticity up to a maximum rolling moment of 120 Nm even in the case of a continuous stress of up to 3 million load alternations without having to dispense with the advantages of the artificial foot known from the DE-PS No. 23 41 887, it is proposed in the DE-Gbm No. 88 04 228.6, that the reinforcing member consist of at least two approximately equally long, superimposed leaf springs which are configured in such as way as to correspond to the rolling profile of the foot. This foot possesses a great mobility in the metatarsophalangeal joint, and an elastic yielding springing both in the forefoot as well as within the calcaneal area, the rolling resistance being predetermined so as to be unchangeable over a relatively long period. Over and above that, a simple adaptation to different heel heights is possible and a maximum of wearing comfort is ensured for the person using the artificial foot without any readjustment function for retaining a uniform foot rooling function becoming necessary. By supporting the metallic reinforcing member in the calcaneal portion of the molded foot part, an increased strength of the molded foot part is achieved without which the requisite resilient and yielding springing means in the forefoot and within the calcaneal area are adversely affected. The simple adaptation to different heel heights is possible by merely changing the tongue disposed in the calcaneal portion of the molded foot part without an alteration of the entire function core being necessary for this in order to achieve an adaptation to different heel heights.
This construction makes it possible in particular that, by means of the resiliently constructed forefoot and metatarsal areas, kinetic energy is absorbed and stored and, in the relief of the rearward movement, is once more fed into the prosthesis in the form of energy being released. This recycling of released kinetic energy into the prosthetic system does demonstrably result in a clear energy relief during the locomotion of the patient. Furthermore, it is now possible to do without ankle joints in the artificial foot, as a result of which a loss of energy arising in the ankle joints due to friction is avoided and the stored energy can be fully utilized again as return energy. The artificial foot is highly resilient within the forefoot and metatarsal area and is capable of absorbing a maximum rolling moment of 120 Nm. This maximum rolling load also determines the dimensioning of the springs.